Automotive proton exchange membrane fuel cells (“PEMFCs”) (a fuel cell used in, for example, an automobile, truck, or other moving vehicle) must meet the needs of a varying dynamic load. During normal operation, the potential of an anode of an automotive PEMFC will be near the reversible hydrogen potential but the cathode potential will vary as the potential of the PEMFC changes to match the variable power demands that are placed on the PEMFC. A typical electrode of an automotive PEMFC includes a metallic electro-catalyst such as nanoparticles of a noble metal (e.g. platinum) supported on carbon and an ionomer binder (e.g. a perfluorinated polymer such as a NAFION polymer). Under dynamic PEMFC operating conditions, the cathode performance can deteriorate. The dynamic conditions generally involve an electrical potential that swings from a low value to a high value and back again. At high values of the potential, nanoparticles of platinum catalyst at the cathode dissolve rapidly. The dissolved nanoparticles of platinum may precipitate as larger sized particles of platinum. Trading the smaller sized nanoparticles of platinum for larger sized particles of platinum reduces the active surface area of the cathode catalyst, which brings with it a gradual degradation of the fuel cell electrode(s) and, consequently, deterioration in fuel cell performance. At low values of potential, water generated by the oxygen reduction reaction of the cathode increases the hydrophilicity of the catalyst layer. The increased hydrophilicity of the catalyst layer tends to inhibit facile gas transport. Furthermore, ionomer binder undergoes long-term relaxation process in the presence of water, which can change the initial optimized three—phase interface (catalyst, ionomer and catalyst supporting materials). The problems associated with a reduction in surface area of the electrocatalyst in combination with problems brought on by water generation are believed to facilitate the degradation of the PEMFC, resulting in deterioration of fuel cell performance.
The US DOE EERE (“United States Department of Energy Office of Energy Efficiency and Renewable Energy”) has developed a PEMFC engine drive cycle tests that are intended to simulate the performance degradation of PEMFCs in automotive applications. These tests typically involve cycling the power density continuously from low values to high values. Table 1 summarizes voltages and their associated duration for a drive cycle profile suggested by the US DOE EERE. In such a DOE test protocol, the performance of fuel cells for vehicular applications can be assessed and compared with a U.S. DOE performance target
TABLE 1StepDuration (in seconds)Voltage (in volts)115Open circuit voltage2250.83200.754150.885240.806200.757150.888250.809200.7510150.8811350.8012200.6013350.651480.8815350.7516400.88This protocol was not intended to simulate everything that can happen to an automotive fuel cell during normal operating conditions. For example, the protocol does not simulate fuel cell behavior under start/stop conditions. However, such a protocol may provide valuable insight about fuel cell durability for automotive-type transients.
The US Council for Automotive Research (USCAR) also has developed accelerated stress tests (ASTs) and polarization protocols for PEMFC to shorten the time required to address durability issues for all drive cycles. An exemplary AST for a cathode expected to be used for an automotive application consists of a triangle sweep cycle at 50 mV/sec between 0.6 V to 1.0 V for 30,000 cycles (16 seconds per sweep) for a PEMFC operating at a temperature of 80° C. with the anode flowing hydrogen gas at 100% relative humidity and the cathode flowing nitrogen gas at 100% relative humidity. The test is meant to simulate the performance of an automotive PEMFC operating in an automobile or other vehicle over the 5,000 hours of fuel cell drive time. Polarization curves are recorded at intervals during the potential cycling in order to follow the performance of the PEMFC.